In recent years, there is an increasing demand for reliability in semiconductor devices in which a semiconductor element is mounted on a substrate. In particular, there is a strong demand for an increase in reliability of a joining portion (hereinafter referred to as “joint reliability”) between the semiconductor element and the substrate, which have a large difference in thermal expansion coefficients.
In general, silicon (Si) or gallium arsenide (GaAs) having an operating temperature of from 100° C. to 125° C. has hitherto been used for the semiconductor element. A joining material to be used for the joining portion between the semiconductor element and the substrate is required to have crack resistance against repetitive thermal stress resulting from the difference in thermal expansion coefficients between the semiconductor element and the substrate, have such heat resistance (high melting point) as to respond to multi-stage joining at the time of assembly, and have a property of not contaminating the semiconductor device. In order to satisfy such requirements, for example, lead-containing solder typified by 95Pb-5Sn solder (containing 95 mass % of Pb and 5 mass % of Sn) is mainly used as the joining material when Si is used for the semiconductor element. Alternatively, for example, Au-containing solder typified by 80Au-20Sn solder (containing 80 mass % of Au and 20 mass % of Sn) is mainly used as the joining material when GaAs is used for the semiconductor element. However, the 95Pb-5Sn solder, which contains large amounts of hazardous lead (Pb), has a problem from the viewpoint of reducing environmental load. In addition, development of alternatives to the 80Au-20Sn solder is strongly desired from the viewpoint of rising prices of noble metals and their reserves.
In addition, from the viewpoint of energy saving, as next-generation devices, semiconductor devices using silicon carbide (SiC) and gallium nitride (GaN) for semiconductor elements have been actively developed. Those semiconductor devices each have an operating temperature of 175° C. or more from the viewpoint of reducing power loss, and the operating temperature is said to be increased to 250° C. in the future. In order to respond to such high operating temperature, attention has been focused on a sinterable metal or a joining material called metal paste containing nano-sized or micro-sized metal particles and an organic solvent (for example, see Patent Documents 1 and 2). In the joining material, the organic solvent is decomposed at the time of heat treatment, and thus the metal particles are sintered with each other to form a joining portion. After the sintering (joining), the joining portion has a temperature limit comparable to the melting point of the metal particles (e.g., 960° C. in the case of silver). The organic solvent decomposes at from about 200° C. to about 300° C. depending on the kind of organic solvent. Therefore, objects to be joined can be joined to each other at a temperature at which the objects are not degraded. Further, after the joining, high heat resistance can be achieved. As the metal particles, silver is generally used from the viewpoint of heat release properties, oxidation resistance, and cost. Other than silver, gold, copper, or nickel is used as the metal particles. However, gold, copper, and nickel are inferior to silver in cost, in a reduction in sintering density due to oxidation, and in a reduction in sintering density due to oxidation and heat release properties, respectively.
However, silver is a metal easily sulfurized. Although a semiconductor device manufacturing plant often includes a clean room, which is a region in which temperature and humidity are controlled, in developing countries, the air environment is poor, and silver is sometimes sulfurized to be discolored owing to automobile exhaust gas or the like before a member is carried into a clean room. In addition, when a clean room has an insufficient structure, silver is sulfurized or oxidized after being affected by the air environment even in the clean room, resulting in a remarkable reduction in yield at the time of mass production.
In view of the foregoing, as a joining material that is free of silver and provides a joining portion having a high temperature limit, there has been proposed a joining material that contains Au and Sn as main components and has added thereto a metal, for example, Pt (for example, see Patent Document 3).
Meanwhile, there is also a demand for an increase in mass production properties of semiconductor devices. Herein, a semiconductor device having good mass production properties mainly means that, in the production of the semiconductor device, a facility in association with joining is simple, storage of a member is easy, the number of steps is small, and talk time is short.
The mass production properties of a semiconductor device depend on the form of the joining material to be used. Paste forms, foil forms, sheet forms, or the like have generally been known as the form of the joining material.
A joining material in a paste form contains a flux, and hence joining can be performed in an atmospheric atmosphere. However, the joining material in a paste form contains a solvent, and hence must be stored in a refrigerator. Further, a printing device for printing the joining material in a paste form and a printing step involving using the printing device are required. Therefore, every time the size of the semiconductor element changes, a dedicated printing plate or the like is required. Moreover, even when the joining material in a paste form itself has low cost, the overall cost is increased owing to the control of the printing step and the use of the printing device. In addition, a joining portion obtained through use of the joining material in a paste form generally has low heat resistance, and hence the thickness of the joining portion must be increased in order to relax shear stress generated from a difference in thermal expansion coefficient between members. However, when the thickness of the joining portion is increased, voids are liable to be generated in the joining portion, and also heat release properties are reduced. When the joining material as described in Patent Document 1 is used, a joining portion excellent in heat resistance is obtained. However, in order to increase joint reliability and heat release properties, the denseness of the metal particles must be increased through pressurization. Accordingly, an expensive high temperature furnace having a pressurization mechanism is required, resulting in an increase in cost.
When a joining material in a foil form or a sheet form is used, joining can be performed with only a simple mounting operation without the necessity for the printing step. However, a mounting device for performing the mounting operation is required. In addition, the joining material in a foil form or a sheet form is free of a flux, and hence the joining must be performed in a reducing gas, resulting in an increase in the overall cost owing to the use of an expensive high temperature furnace for use with a reducing gas.
Meanwhile, a method of forming a joint film through vapor deposition can eliminate the need for the mounting or printing of the joining material, and hence can achieve a reduction in the number of steps. In addition, although there is an impression that vapor deposition has higher costs than printing, the cost per semiconductor element is inexpensive as compared to that in the case of printing when the vapor deposition is performed on a semiconductor wafer in a mass production stage. In addition, vapor deposition also has the advantages of fewer variations in film thickness and high stability.